- Email: [email protected]
Energy selection spectroscopy in the triplet state: High resolution phosphorescence polarization spectra of benzo[a]phenazine
Journal ofLuminescence 24/25 (1981)497—498 North-Holland Publishing Company
497
ENERGY SELECTION SPECTROSCOI~Y[N THE TRIPLET STATE: HIGH RESOLUTION PHOSPHORESCENCE POLARIZATION SPECTRA OF BENZO[a]PRENAZINE Georg W. Suter and Urs P. Wild Physical Chemistry Laboratory Swiss Federal Institute of Technnlngy CH—8002 ZUrich SWITZERLAND
5o excitation The technique of energy selection utilicing direct T was used to obtain highly resolved phosphorescence 1-. spectra of benco— [aiphenacine in glassy matrices at 2-4K. In a heavy atom snlvent a specific enhancement of the totally symmetric vibrational bands was found. For the first time highly resolved spectra of the degree of phosphorescence polarication were obtained, using a combination of the energy selection and the photoselection techniques. [NTRODUCTION The interpretation of the electronic spectra of organic molecules is greatly simplified by knowledge of the emission spectra polarication. The method of photoselection, as generally employed, provides insufficient wavelength resolution to separate individual vibronic bands within an electronic transition. The study of doped crystals gives excellent wavelength resolution but is restricted to a relatively small class of suitable guest-host systems. AL combination of the energy selection and photoselection techniques enables highly resolved phosphorescence spectra to be obtained. The energy selection effect is restricted to the electronic transition which is initially excited [1,2]. Thus, in order to obtain highly resolved phosphorescence spectra, cessary to excite directly into the very weak T 1 — Sq absorption band.
it is ne-
In a heavy atom solvent such transitions show the following behaviour: (i) The overall electronic transition moment is significantly increased. The absorption spectrum is more intense and the phosphorescence lifetimes are shortened (ii) The 0-0 band and the totally symmetric vibrations are strongly enhanced relative to the non—totally symmetric modes (Kearns effect [3]). (iii) The orientaHon of the T1 — S~transition moment is dependent on the local heavy atom environment of the chromophor. Information about the symmetries of the vibrations can thus be gained either by comparing the phosphorescence spectra obtained in a normal and in a heavy atom containing solvent, or by directly measuring the polarieation degree in a non-heavy atom solvent. In this ~nper both methods are applied to ascertain the symmetry of some vibrational bands in the phosphorescence spectrum of benco [a] phenacine. RESULTh The energy selection phosphorescence spectra obtained in n-butylbromide (BuBr) and in 2—methyltetrahydrofuran (2—MTRF) are shown in Fig. 1. The excitation source was a tunable Rhodamin fIG ew dye laser. In order to suppress stray-light and all short-lived emissions both the excitation and emission beams were chopped with opposite phase. shifting
0 022—2313/81 /0000—0000/$02.75 © North-Holland
0. lt’utcr,
495
I I’. IC//b / Lstrrgr sc/reline s/a’stro$ropr /tt I/sr Ftp/st slate
irs 2-MIfF gap7271~n-1
I~__ _ _ _ _ _ _ _ _ _ _ _ I .~IJ°LJ~J\J&fi J~Jj\~NJkJ\
I
~t
17500
WAV[NUM8ERS
IM—t I ‘155~~
585
‘
5~l WA5ELENGTH
Fig. 1
509
Fig. 2
the excitation wavelength results in a correspondingly shifted phosphorescence spectrum with identical line structure. A full vibrational analysis of bunco [a[ phenacine in BuBr is given in 4]. A comparison of the two spectra clearly shows significant changes in the relative intensities of the individual vihronic bands. The bands marked in Fig. 1 are not enhanced by the heavy atont solvent and can therefore he assigned to non—totally symmetric tnodes (Table I). Fig. 2 shows a part of the spectrom of phosphorescence polarization degree io 2—MTIIF. Effects such as sample geometry and mechaoical strain in the super— cooled glassy solvent can seriously affect the absolute values of the polarization degree. CONCLUSION
Table I
Symmetry assignment of the phosphorescence spectra of heozo a pheoazioe
i’he vibrational analysis, based on the IKearns effect and on direct polarization Transition measurements, yields consistent results, cm~ which are in substantial agreement with 0—270 a “conventional” analysis [ii. The coin— 0—312 parisoo of heavy atom and non—heavy abnt 0—305 spectra, obtained by energy selection, 0—480 provides not only high resolution hut also 0—502 information about the symmetry of the vi— 0—545 brational hands. 0-571)
0—rio
Polari— cation a” a’ a’ a” a’ a” a”
Kearos effect a” a’ a’ a” a’ a” a”
a’
a’
according I ii a’ a’ a’ a” a’ a” a” a’
RE FEB EN CES
I~l
Alshita, E. I,, Personov, R. I., Kharlamow, H. M. , Chem. Phys. Lett. 40 (1970) 110.
(2]
Griesser,
[3]
Giaehino, G. G.
[41
Breoner,
II. 5., Wild, U. P., J. Chesa. Phys. 73 (1980) 4715.
K.
,
,
Kearos, 0. 14.,
Huziewiec, Z.
,
J.Chem. Phys. 52 (1909) 2901. J. them. Phys. 53 (1970) 3880.
Suter, G. IV.
,
Wild, U. P.
,
(‘hem. Phys. accepted for publication.
497
ENERGY SELECTION SPECTROSCOI~Y[N THE TRIPLET STATE: HIGH RESOLUTION PHOSPHORESCENCE POLARIZATION SPECTRA OF BENZO[a]PRENAZINE Georg W. Suter and Urs P. Wild Physical Chemistry Laboratory Swiss Federal Institute of Technnlngy CH—8002 ZUrich SWITZERLAND
5o excitation The technique of energy selection utilicing direct T was used to obtain highly resolved phosphorescence 1-. spectra of benco— [aiphenacine in glassy matrices at 2-4K. In a heavy atom snlvent a specific enhancement of the totally symmetric vibrational bands was found. For the first time highly resolved spectra of the degree of phosphorescence polarication were obtained, using a combination of the energy selection and the photoselection techniques. [NTRODUCTION The interpretation of the electronic spectra of organic molecules is greatly simplified by knowledge of the emission spectra polarication. The method of photoselection, as generally employed, provides insufficient wavelength resolution to separate individual vibronic bands within an electronic transition. The study of doped crystals gives excellent wavelength resolution but is restricted to a relatively small class of suitable guest-host systems. AL combination of the energy selection and photoselection techniques enables highly resolved phosphorescence spectra to be obtained. The energy selection effect is restricted to the electronic transition which is initially excited [1,2]. Thus, in order to obtain highly resolved phosphorescence spectra, cessary to excite directly into the very weak T 1 — Sq absorption band.
it is ne-
In a heavy atom solvent such transitions show the following behaviour: (i) The overall electronic transition moment is significantly increased. The absorption spectrum is more intense and the phosphorescence lifetimes are shortened (ii) The 0-0 band and the totally symmetric vibrations are strongly enhanced relative to the non—totally symmetric modes (Kearns effect [3]). (iii) The orientaHon of the T1 — S~transition moment is dependent on the local heavy atom environment of the chromophor. Information about the symmetries of the vibrations can thus be gained either by comparing the phosphorescence spectra obtained in a normal and in a heavy atom containing solvent, or by directly measuring the polarieation degree in a non-heavy atom solvent. In this ~nper both methods are applied to ascertain the symmetry of some vibrational bands in the phosphorescence spectrum of benco [a] phenacine. RESULTh The energy selection phosphorescence spectra obtained in n-butylbromide (BuBr) and in 2—methyltetrahydrofuran (2—MTRF) are shown in Fig. 1. The excitation source was a tunable Rhodamin fIG ew dye laser. In order to suppress stray-light and all short-lived emissions both the excitation and emission beams were chopped with opposite phase. shifting
0 022—2313/81 /0000—0000/$02.75 © North-Holland
0. lt’utcr,
495
I I’. IC//b / Lstrrgr sc/reline s/a’stro$ropr /tt I/sr Ftp/st slate
irs 2-MIfF gap7271~n-1
I~__ _ _ _ _ _ _ _ _ _ _ _ I .~IJ°LJ~J\J&fi J~Jj\~NJkJ\
I
~t
17500
WAV[NUM8ERS
IM—t I ‘155~~
585
‘
5~l WA5ELENGTH
Fig. 1
509
Fig. 2
the excitation wavelength results in a correspondingly shifted phosphorescence spectrum with identical line structure. A full vibrational analysis of bunco [a[ phenacine in BuBr is given in 4]. A comparison of the two spectra clearly shows significant changes in the relative intensities of the individual vihronic bands. The bands marked in Fig. 1 are not enhanced by the heavy atont solvent and can therefore he assigned to non—totally symmetric tnodes (Table I). Fig. 2 shows a part of the spectrom of phosphorescence polarization degree io 2—MTIIF. Effects such as sample geometry and mechaoical strain in the super— cooled glassy solvent can seriously affect the absolute values of the polarization degree. CONCLUSION
Table I
Symmetry assignment of the phosphorescence spectra of heozo a pheoazioe
i’he vibrational analysis, based on the IKearns effect and on direct polarization Transition measurements, yields consistent results, cm~ which are in substantial agreement with 0—270 a “conventional” analysis [ii. The coin— 0—312 parisoo of heavy atom and non—heavy abnt 0—305 spectra, obtained by energy selection, 0—480 provides not only high resolution hut also 0—502 information about the symmetry of the vi— 0—545 brational hands. 0-571)
0—rio
Polari— cation a” a’ a’ a” a’ a” a”
Kearos effect a” a’ a’ a” a’ a” a”
a’
a’
according I ii a’ a’ a’ a” a’ a” a” a’
RE FEB EN CES
I~l
Alshita, E. I,, Personov, R. I., Kharlamow, H. M. , Chem. Phys. Lett. 40 (1970) 110.
(2]
Griesser,
[3]
Giaehino, G. G.
[41
Breoner,
II. 5., Wild, U. P., J. Chesa. Phys. 73 (1980) 4715.
K.
,
,
Kearos, 0. 14.,
Huziewiec, Z.
,
J.Chem. Phys. 52 (1909) 2901. J. them. Phys. 53 (1970) 3880.
Suter, G. IV.
,
Wild, U. P.
,
(‘hem. Phys. accepted for publication.